This invention is concerned generally with medical electronics, and, more particularly, with ventilator patient monitors.
A ventilator is used to provide supplemental or complete respiration for a patient when the patient is unable to maintain sufficient respiration without assistance. The majority of ventilators used for supplemental or complete patient respiration are of the type known as intermittent positive pressure breathing (IPPB) ventilators. An IPPB ventilator provides positive pressure to force air into the lungs of the patient to accomplish inspiration. Inspiration is then followed by expiration by the patient without assistance. This cycle is then repeated at a predetermined rate.
The typical IPPB ventilator system comprises a gas supply, a pumping mechanism, interconnection tubing to the patient, an expiration valve with an input port and an output port, and either an electrical or a pneumatic control line from the pumping mechanism to the expiration valve. The tubing includes a tee connector and three lengths of tubing. One tube is connected between the pumping mechanism and one of the two coaxial stems of the tee connector, a second tube is connected between the input port of the expiration valve and the second of the coaxial stems of the tee connector, and a third tube is connected between the third stem of the tee connector and the patient.
The expiration valve is a low pressure pneumatic valve which typically experiences a maximum internal gas flow pressure of 0.9 to 1.2 PSI. Control of gas flow through the valve is performed by an electrically or pneumatically activated diaphragm in the flow stream to open and close the valve. The majority of the IPPB ventilators utilize a pneumatic valve control signal. In the pneumatically controlled expiration valve, the diaphragm is analogous to a small balloon in a pipe which is inflated by the pneumatic signal from the ventilator pumping mechanism to provide valve closure. The diaphragm deflates when the pneumatic signal is relaxed, allowing the valve to open.
The effectiveness of supplemental or complete respiration can be determined by observing the blood gas parameters of the patient. Adequate ventilation of the patient is indicated if the blood contains appropriate levels of oxygen and carbon dioxide. However, it has not been feasible to monitor blood gases continuously.
Although periodic blood gas measurements can still be taken, a more immediate indication of loss of ventilation is needed, as ventilation loss for more than one minute can be fatal. Consequently, most ventilator monitors measure the volume of gas used to ventilate the patient. The measured gas volume has been either the volume of gas supplied to the patient or the volume of gas expired by the patient. Expiratory monitoring avoids the adverse effects of loss of patient ventilation through leaks in the gas flow system. Leaks may cause some of the expired gas volume to escape detection by the monitor; however, the operator is assured that the patient receives at least as much of the ventilator output as is detected. On the other hand, if the output of the ventilator is monitored before the gas reaches the patient, there is no positive indication as to the minimum gas volume that reaches the patient since this gas flow system may have inherent leaks between the monitor and the patient.
Expiratory gas volume is measured by connecting the monitor to the output port of the expiration valve. The expiration valve ideally will permit the free-flow of expired gas from the patient to the monitor, and block the flow to the monitor of the inspiratory gas from the ventilator to the patient. There are many reasons why the expiration valve may leak, e.g. a defective diaphragm, a defective valve seat, a leak in the pneumatic valve control line so that the valve diaphragm is not fully closed, or a build-up of patient secretions in the valve. Secretions from the patient build-up between the expiration valve diaphragm and the valve seat causing gas leakage during inspiration. These secretions prevent complete closure of the expiration valve since the pressure applied to inflate the expiration valve diaphragm is typically only 1.5 to 3.0 PSI. This range of pressures is not sufficient to break through the secretions to achieve closure of the valve pipe against a back pressure that is nearly equal to the diaphragm control pressure. When leakage occurs, the volume of gas measured by the monitor will contain gas expired by the patient and that which leaked through the valve during inspiration. Therefore, the measured gas volume will be greater than that expired by the patient. In the extreme case where the valve leaks continuously in a fully open condition, the volume of gas measured by the monitor indicates that normal gas volume is being delivered to the patient, when in fact the patient has received none of the measured gas volume. Heretofore known monitors had no mechanism for distinguishing when the received gas was from the patient or from the ventilator. These monitors relied on the possibly leaky expiration valve to block the inspiratory gas from the ventilator.
Since the expiration valve must be sterilized, and the pneumatically controlled valves must be operational from the ventilator low pressure control line, only limited improvements are possible in the expiration valve itself. Many hospitals prefer to use disposible items wherever possible, thus placing a cost limitation on the expiration valve, in addition to the aforementioned limitations. The failure of the expiration valve is not readily detectable with the present monitors. Thus, a blood gas analysis is the method relied upon by the doctor and his staff to insure the proper ventilation of the patient.